The screed assembly for an asphalt paving machine is employed to level and grade the asphalt concrete material that is deposited by the paving machine on the roadway, as well as to provide initial compaction for the newly applied pavement surface. Consequently, the screed assembly establishes the initial thickness of the layer of paving material that is placed on the roadway by the paving machine. In order to accomplish this, the screed assembly is typically adapted to be towed by or pulled behind the tractor unit of the paver. The screed or screed assembly is usually attached to the tractor by means of a pair of side arms or pull arms at a single point (the tow or pull point) on either side of the machine. The attachment is typically a pivotal connection that allows the screed to float freely on the mat of paving material that has been deposited on the roadway. This allows the screed to average out changes in grade that are spanned by the wheels or crawler assemblies of the paver tractor.
The angle assumed by the lower surface of the screed as it floats on the mat of paving material is referred to as the angle of attack. The forces on the screed must be in equilibrium for the screed to remain at a constant angle of attack as it is towed by the tractor. There are two primary forces that constantly act on the paver screed as the paver places the paving material on the roadway, and changes in these forces change the angle of attack assumed by the screed. The first force acting on the screed is the towing force of the tractor, which varies as the speed of the tractor changes. The second force is due to the head of paving material deposited behind the tractor pushing against the screed. As the amount of such material deposited behind the tractor changes, the force acting on the screed also changes. When a change in forces acting on the screed occurs, the screed will rise or fall, changing its angle of attack, and this will change the thickness of the mat being placed. The screed will continue to react to a change in forces acting against it until it reaches a new equilibrium angle of attack in which the forces are again in balance.
It is known that the service life of an asphalt surface layer on a roadway depends largely on the reduction of voids therein to the smallest possible volume. The determining factor in void volume is the degree of compaction imparted to the asphalt mat. It is generally considered desirable that the surface layer of an asphalt roadway have a degree of compaction of at least about 96-98%. In order to achieve this degree of compaction, therefore, the air voids in a mat or layer of asphalt must be reduced so as to occupy no more than about 2-4% of the volume of the asphalt mat.
The amount of compaction imparted to the asphalt mix is a function of many variables. The properties of the mix itself are very important. The temperature, amount of asphalt cement, and moisture in the mix all affect the forces required to compact the mix. In practice, it has generally been necessary, in order to achieve the desired degree of compaction, to provide a screed assembly on the paving machine having a front-mounted hydraulically operated tamper or compactor bar to precompact the asphalt mat, followed by a leveling screed plate that provides additional compaction. The tamper that is typically mounted on the screed assembly is provided with an oblique leading face that compresses the asphalt concrete material as it passes thereover, and a hydraulic ram that compacts it beneath the bar. The following leveling screed plate then acts to close and smooth the asphalt surface, although the compaction obtained by use of this screed assembly will typically be no greater than about 86-92%. In order to obtain the desired degree of compaction, it is generally necessary to provide a separate compaction step by making one or more passes over the partially-compacted asphalt mat with a compacting roller.
It is also known to adapt the leveling screed plate for vibration in order to enhance its leveling and smoothing action, and to permit it to provide an additional measure of compaction to the asphalt mat. A typical vibratory screed for an asphalt concrete paving machine is provided with an elongate shaft, means for rotating the shaft, and one or more eccentric weights that are mounted on the shaft so that vibration will be created upon rotation thereof. One example of such a vibratory screed assembly is described in U.S. Pat. No. 3,545,349 of Otterman et al. The Otterman screed includes a front plate that is adjustable with regard to its angle of inclination, a tamper or compactor bar that is arranged immediately behind the plate, and a smoothing screed plate behind the tamper. The tamper is suspended at both ends from an eccentric drive which imparts vibration to the tamper through a motor-driven shaft on which a cam disc is attached, in order to provide a tamping action that imparts a measure of compaction of the asphalt mat. In addition, the screed plate is vibrated by means of a rotary driven shaft carrying eccentrically-mounted weights for generating pulsating vibratory forces in all directions in planes extending perpendicular to the shaft.
The frequency of vibration and the amplitude of vibratory forces applied by such a vibratory mechanism that is installed on a screed assembly will affect the amount of compaction imparted through the assembly to the asphalt mat. The frequency of vibration of a rotary-driven shaft carrying eccentrically-mounted weights is controlled by the rotary speed of the vibrator shaft, and the amplitude of vibratory forces is controlled by the size and location of the eccentric weights on the shaft. The amount of compaction imparted by a screed plate equipped with such vibratory mechanism is also affected by the size and mass of the screed plate and the speed of the paver screed. The faster the paver moves, the less time the screed will reside over any particular area in the new pavement mat, and thus, the less the degree of compaction imparted.
Another variable in the amount of compaction imparted by a vibratory screed assembly is the efficiency by which such assembly operates. Generally, the greater the amount of generated forces that are applied in the vertical direction, the greater the amount of compaction imparted to the pavement mat. In addition, the greater the amount of generated forces that are applied to the pavement mat, the less is the need for subsequent rolling or compacting operations. However, the vibration imparted to the screed plate in a vibratory screed assembly is normally limited to no more than twice the total weight of the assembly. A greater resultant force would cause the screed assembly to bounce or jump on the asphalt mat, which would result in damage to the asphalt surface and the screed assembly.
The typical vibratory screed does not efficiently transmit the forces generated by the vibratory assembly to the asphalt pavement, because the forces transmitted by a rotating eccentrically mounted weight are not restricted to the vertical direction, but also include forces acting in the travel direction of the paver, or obliquely thereto. Because the typical vibratory screed does not efficiently transmit compacting forces to the asphalt pavement, subsequent rolling operations are required to achieve the desired degree of compaction. Furthermore, because the asphalt mat is not properly compacted as it is applied, the subsequent rolling operations may cause roll-out of asphalt concrete material at the edges of the roadway. Such roll-out, where asphalt concrete material at the edge of the roadway is squashed out laterally under the influence of the compacting force of the roller, may create an uneven edge on the asphalt mat and may cause the edge to crumble or fail. This may create considerable difficulty in matching the density of asphalt mats that are joined longitudinally, such as at the center of a road. In addition, because the rolling machine is usually not the same width as the paving machine, it is sometimes difficult to provide uniform compaction to the entire width of the asphalt mat being laid.
In addition to inefficiently transmitting compacting forces to the pavement, the typical vibratory screed can have other deleterious effects on the paving operation. If vibrations generated by the vibratory screed are transmitted through the pull arms to the tractor of the paver instead of to the pavement, damage to the pivotal connection of the screed assembly or to the tractor itself may result. Furthermore, the transmission of such vibrations to the tractor can affect the deposit of asphalt material by the paver, and in some cases may even be felt by the paver operator.
U.S. Pat. No. 4,493,585 of Axer describes a vibratory screed mechanism that attempts to address some of the functional deficiencies in typical vibratory screed assemblies noted above. However, Axer's solution is a complicated, multi-component mechanism that includes five principal subassemblies. First, Axer employs a leveling blade, followed by a vertically moveable ramming bar and a first leveling screed plate. Located to the rear of the first leveling screed plate is a compactor bar, followed by second leveling screed plate that is mounted on a lower plane than the first plate. The ramming bar is operatively connected to an eccentric drive, which provides a vertical tamping action at the leading face of the first leveling screed plate. Between the first leveling screed plate and the second leveling screed plate, the compactor bar is arranged for vertical compacting action, and both screed plates may also be provided with a vibrator device. Axer's device also includes a complicated arrangement for transmitting vibratory forces from the eccentrically weighted drive shaft to the asphalt pavement mat, including push rods, followers, guide rods and springs, as well as a pressure beam.
It would be desirable, however, if a more efficient vibratory screed could be developed for an asphalt paving machine that would transmit greater compacting forces to the asphalt pavement mat without requiring a complicated force-transmitting mechanism, or an increase in the mass of the screed or the power supplied to the vibratory mechanism. It would also be desirable if a vibratory screed could be developed that would be capable of imparting significant vibratory forces to the asphalt pavement mat without requiring the use of a precompacting tamper or a plurality of ramming and compacting bars. In addition, it would be desirable if a vibratory screed of relatively simple design could be developed that is susceptible to adaptation for use on paving machines of various widths. It would also be desirable if a vibratory screed assembly could be developed that would permit compaction of the asphalt mat to the desired density as it is laid, so that subsequent rolling operations could be minimized or eliminated. Finally, it would be desirable if an improved vibratory screed could be developed for use in connection with an asphalt paving machine that would minimize the vibratory forces transmitted to the tractor of the paving machine.